Alloy 718 is the most widely used iron-nickel-base superalloy. This Alloy possesses excellent strength, low cycle fatigue and creep behavior in addition to outstanding corrosion resistance in many environments. In addition, this Alloy is weldable and highly fabricable. Because of this, Alloy 718 has been used extensively in the aerospace industry and is being used more extensively in the nuclear power industry where high strength and excellent stress corrosion cracking resistance are important design criteria.
Since Alloy 718 was originally utilized in the aerospace industry, present heat treatment techniques for this Alloy were designed to optimize the high temperature mechanical properties required for air-breathing jet engine environments. The material requirements within the nuclear power industry are significantly different from those in aerospace applications, and thus, different heat treatment techniques are necessary. In particular, even though Alloy 718 has performed relatively well in light water reactor (LWR) environments, some failures have occurred, and it has been determined that these failures are the result of intergranular stress corrosion cracking (IGSCC). Prior to this time, to the best of the inventors' knowledge, there have not been any systematic studies to evaluate the effect of heat treatment, and therefore microstructure, on the intergranular stress corrosion cracking performance of Alloy 718 in light water reactor environments, and the development of a heat treatment method to optimize the performance of this Alloy in nuclear power reactor environments.
Even though a modified post-weld heat treatment was developed for Alloy 718 for used in liquid metal fast breeder reactors, heretofore conventionally heat treated Alloy 718 is being used in light water reactor environments with failures occurring as a result of intergranular stress corrosion cracking (IGSCC).
Accordingly, there is a need for a method to increase the IGSCC resistance of stock or unwelded Alloy 718 in light water reactor environments and in hydrogenated aqueous environments such as pressurized water reactors.